The process of the present invention relates to the refining of lubricating oils and derivatives therefrom.
In the past, lubricating oils and derivatives therefrom have been refined by employing a finishing treatment using clay or a combination of sulfuric acid and clay. In many cases, the feedstock had been solvent or catalytically dewaxed and/or solvent extracted. Such finishing treatments provided a treated product that possessed improved color and odor and, in many cases, improved stability to light and oxidation. In general, stability to oxidation and to light has been maintained by the addition of one or more chemicals to the finished product. In recent years, the quality of lubricating oils has been improved by a finishing treatment comprising a relatively severe hydrogenation treatment.
One of the important applications of hydrotreating a lubricating oil distillate is the hydrotreating of mineral oils to produce "white oils." A white oil is a highly refined lubricating oil fraction which is colorless, odorless, and tasteless; and it must be essentially free of aromatic hydrocarbons. It must have a color of +30 Saybolt and must possess a low absorbance of ultraviolet light. Typically white oils can be used for cosmetics and certain medicinal purposes.
The hydrogenation treatment of a lubricating oil fraction to produce a white oil, in general, comprises a two-stage process. In the first stage of a typical process, the selected lubricating oil fraction is desulfurized over a sulfactive hydrogenation catalyst under relatively severe hydrogenation conditions, and the effluent from this first stage is contacted in a second stage with hydrogen under relatively mild conditions with a hydrogenation catalyst comprising a platinum group metal on a non-acidic or a weakly-acidic support.
For instance, U.S. Pat. No. 3,841,995 (Bertolacini et al.) the teachings of which are incorporated herein by reference, discloses a two-zone hydrogenation process for producing a colorless mineral oil by hydrogenating and desulfurizing a lubricating oil distillate with a sulfactive hydrogenation catalyst in a first reaction zone and by contacting the first-reaction zone effluent in a second reaction zone with hydrogen and a catalyst containing a Group VIII noble metal deposited on a large-pore-diameter alumina having a surface area of 150-500 m.sup.2 /g and an average pore diameter of 100-200 Angstroms. The lubricating oil distillate feed in the Bertolacini et al. process is initially dewaxed and may also be solvent-extracted to reduce its aromatic hydrocarbon content.
U.S. Pat. No. 4,269,695 (Silk et al.) discloses a process for reclaiming wax-contaminated lube base stock oils which process consists of contacting the oil with hydrogen in a single reaction zone at a temperature of 500.degree. to 675.degree. F., a hydrogen partial pressure of about 100 to 1500, and at a space velocity higher than 2 and up to about 10. The catalyst used in the reaction zone is a crystalline alumino silicate zeolite having a silica to alumina ratio of at least 12 and a Constraint Index of about 1 to about 12, i.e. a ZSM-5 type zeolite.
U.S. Pat. No. 4,263,127 (Rausch et al.) relates to the preparation of food grade white mineral oils of suitable viscosity in high yield from a mineral oil distillate of suitable lubricating oil viscosity. The Rausch et al. process comprises contacting the distillate with hydrogen in three catalytic stages to yield a refined lubricating oil from which white mineral oil is recovered. The first reaction stage employs hydrocracking conditions. Subsequent reaction stages employ hydrogenation conditions, first with a sulfur-resistant hydrogenation catalyst and finally with a platinum group metal-containing selective hydrogenation catalyst, optionally activated with a halogen. The last selective hydrogenation step is carried out at from about 450.degree. F. to about 650.degree. F., and a pressure in the range from about 1000 psig to about 5000 psig. The selective hydrogenation catalyst comprises a Group VIII platinum group metal on a support comprising calcined or activated alumina.
U.S. Pat. No. 4,325,804 (Everett et al.) also relates to the preparation of high quality, e.g., high viscosity index, base lubricating oils and white oils, particularly food grade white mineral oils, of suitable viscosity in high yield from a mineral oil distillate of suitable lubricating oil viscosity. The Everett et al. process comprises contacting the distillate with hydrogen in four catalytic stages. The first reaction stage employs hydrocracking conditions. Subsequent reaction stages employ hydrogenation conditions. The second reaction stage preferably employs a sulfur-resistant hydrogenation catalyst and produces a product suitable as a high quality lubricating oil base stock. The third reaction stage preferably employs a sulfur-resistant hydrogenation catalyst to obtain further aromatic saturation. The final stage employs a selective hydrogenation catalyst, optionally activated with a halogen, and produces a product suitable as a white oil, preferably a food grade white oil. Patentees' selective hydrogenation step is carried out at a temperature within the range of 500.degree. F. to 575.degree. F. and a pressure of 2,000 psig. to about 3,000 psig. The selective hydrogenation catalyst comprises a platinum group metal on an activated or calcined alumina.
While the above white oil preparation processes are acceptable, one of the problems arising from the preparation of white oils and particularly medicinal or food grade white oils is the formation of an unsightly waxy haze also known as solid paraffins during long-term low temperature storage. For instance, a refined oil can be prepared which is clear and bright and which has a satisfactory cloud point and pour point but upon storage at a low temperature above the cloud point a wax haze develops which makes the oil aesthetically unattractive and often commercially unacceptable.
Medicinal and food grade white oils must meet stringent U.S. Pharmacopoeia and Food and Drug Administration specifications. These specifications generally preclude blending of on-specification materials with off-specification materials. Thus the refiner has had few options other than to reprocess the off-specification material, for instance, by feeding it to a catalytic cracking unit.
When a refined product fails a haze test, it is particularly expensive for the refiner since the raw material and process costs have been expended to make the product. The amount of haze present e.g. typically less than 2 wt. % wax contaminant must be removed without impairing other properties and meeting the pertinent white oil specifications. In this connection, the Silk et al. process is unacceptable since its conditions produce products which do not meet the aromatics specifications for white oils. Specifically, the temperatures used in the Silk et al. process are too high to ensure hydrogenation of all aromatics. Temperatures above 500.degree. F. change the equilibrium constraints, such that the presence of aromatics is preferred.
In any event there is a need for a process that will remove minor amounts of haze or solid paraffins from medicinal grade white oils or other technical grade white oils without an inordinate yield loss or detrimental effect on the other specified white oil properties.
Accordingly, it is an object of the present invention to provide a process to produce haze-free white oils. Another object is to provide a process for haze removal from a white oil whether technical or medicinal grade, which process provides a minimal yield debit, does not impair the white oils' other pertinent specifications, and remains haze-free for a long-term low temperature storage.